Wounds produce exudate. The composition of this exudate varies depending upon the nature and location of the wound. While an exudate can be generically characterized as an aqueous mixture of proteinaceous materials, it may also contain blood components, etc., and may serve as a growth medium for bacteria.
In clinical practice, the dressing of wounds has traditionally been accomplished by cleansing the injured area and covering it with absorptive, gauze type materials. In this way the wound is kept "clean and dry" throughout the duration of its healing process.
However, there has been increasing acceptance of the view that for optimal wound healing to occur from the standpoint of rate of healing, quality of healing, etc., a moist microenvironment around the wound is preferential as opposed to the "clean and dry" approach. As this acceptance of the moist wound healing theory has grown, wound dressings promoting a moist wound microenvironment have entered the marketplace.
The control of exudate is of prime importance if a moist wound microenvironment is to be maintained. It can be appreciated that if a dressing removes all the exudate that a wound produces, a "dry" wound results which is suboptimal for wound healing. Similarly, if the dressing does not control the level of exudate sufficiently, then an excess "pool" of material develops which may subsequently leak thus soiling clothing and bed linen, and breaching any barrier to bacterial infection.
Ideally, a wound dressing must be adhesive in nature such that it may attach to the wound site. The adhesive utilized must be biocompatible, non-cytotoxic and free of toxic leachable substances as well as have the desired balance of physical properties such as moisture vapor transport rate, tack, long term adhesion properties, etc. Inasmuch as in use the adhesive will be in direct contact with the wound site and surrounding intact area, it must be non-toxic and should elicit no more than a minimal allergenic response.
Additionally, a wound dressing should possess the ability to prevent bacteria from entering the wound from the ambient environment while providing the proper moisture vapor transport rate.
Other aspects such as a dressing's ability to conform to irregular contours of the body are also desirable. This may be accomplished by utilizing elastomeric, flexible, polymeric materials in the construction of the dressing.
Having outlined the major desirable design characteristics of environmental wound dressings it is beneficial to examine the mode of operation of existing wound dressings to appreciate their deficiencies.
Environmental dressings, i.e., dressings which maintain a beneficial microenvironment around a wound, can be categorized into three broad classes: hydrocolloid/gel dressings; film dressings; and foam dressings.
These dressings maintain specific microenvironments, e.g., moisture, temperature, gaseous transport, etc., around a wound by utilizing a variety of physical mechanisms.
Hydrocolloid type dressings as described in U.S. Pat. No. 4,477,325 to Osburn are relatively thick and as a result possess low conformability. The mode of exudate control is by absolute absorption by a gel (hydrocolloid) in the dressing. The ability for moisture to pass through the dressing to the external environment is minimal. On highly exuding wounds the dressing's absorption capacity can be exceeded which leads to leakage and subsequent disruption of the bacterial barrier. Some hydrocolloid compositions dissolve and fall into the wound bed thus requiring time consuming cleaning, which disrupts the wound site, at subsequent dressing changes.
The ability of a film dressing to transport moisture is a function of film thickness and chemical composition. On moderate to high exuding wounds, exudate tends to collect under film dressings and form "pools". This collection of exudate indicates that current polymer film dressings have a moisture vapor transmission rate which is too low to handle the exudate from many wounds. It has been suggested that the "pool" of exudate may increase the risk of bacterial proliferation leading to infection. Similarly, if the "pool" reaches excessive proportions leakage will occur thus breaking the bacterial barrier.
Polymer film dressings as described in U.S. Pat. No. 3,645,835 to Hodgson and U.S. Pat. No. 4,513,739 to Johns are thin and possess high conformability. The wound contacting surfaces are coated with pressure sensitive adhesives carried on the film. The films that are used are liquid impermeable polyurethane elastomers. Thus wound exudate is not allowed to ingress into the film. The sole mode of exudate control is by allowing vapor of the aqueous portion of the exudate to permeate into the polymer film from where it diffuses into the external environment. As the moisture vapor permeability is low, the polymer film's absolute absorption capacity is also low especially when compared to hydrocolloid dressings.
Foam dressings also manage exudate by evaporation of the aqueous portion of the exudate through the dressing to the surrounding environment. Control of this moisture vapor transmission rate is a function of the chemical composition of the foam coupled with the pore structure. Due to their gross pore sizes, foam dressings tend to desiccate wounds resulting in dressings which become brittle and nonconformable during use. These hardened dressings often traumatize the underlying healing wound bed. Furthermore, either special processing and/or a wetting agent is required to make the foam hydrophilic.
Dependent upon the type of foam structure used, exudate is also managed by capillary action into the pores of the structure. Most foam structures used as dressings contain interconnecting pores and thus provide limited bacterial barrier properties because the mean pore diameter exceeds the dimensions of many bacteria. Similarly, such dressings contain pore sizes which are sufficiently large as to fall into the range of sizes into which regenerating tissue will grow. As a result of this, ingrowth of tissue into the dressing's structure occurs thus impeding removal of the dressing and traumatizing the wound site.
Open cell foam dressings are described in U.S. Pat. No. 3,975,567 to Lock and U.S. Pat. No. 3,978,855 to McRae et al. The foam is made hydrophilic by permanently compressing the cells of the microporous foam to form a microporous skin which contacts the wound site. Thus these patents teach against larger pores contacting the wound site and smaller pores away from the wound site as is taught by the present invention. Also, a wetting agent is added to enhance absorption of exudate into the porous structure.
An example of a foam dressing with pores into which regenerating tissue grows is U.S. Pat. No. 3,949,742 to Nowakowski. In this particular invention a thrombogenic open cell reticulated foam is used laminated to a non-porous polyurethane film. The dressing provides a matrix into which fibroblasts and new capillaries can grow. Thus the wound site is traumatized by removal of the dressing.
The present invention maintains the desired level of moisture, temperature and gaseous exchange at the wound site. By the control of these properties, the microenvironment thus produced is the optimal required for healing of the wound. At the same time, the present invention manages exudate, is adhesive, biocompatible, non-toxic, conformable, elastomeric and also provides a bacterial barrier.